A switch-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a dc input voltage into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter and based thereon modifies a duty cycle of the switches of the power converter. The duty cycle is a ratio represented by a conduction period of a switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the switch would be 0.5 (or 50 percent). Additionally, as the needs for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on the microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the switches therein to maintain an output characteristic such as an output voltage at a desired value.
In an exemplary application, the power converters have the capability to convert an unregulated input voltage, such as five volts, supplied by an input voltage source to a lower, regulated, output voltage, such as 2.5 volts, to power a load. To provide the voltage conversion and regulation functions, the power converters include active switches such as metal-oxide semiconductor field-effect transistors (“MOSFETs”) that are coupled to the voltage source and periodically switch a reactive circuit element such as an inductor to the voltage source at a switching frequency that may be on the order of five megahertz.
In some applications of dc-dc power converters, before the power converter is initially turned on, an output capacitor is pre-charged by an external circuit. A requirement in this type of application is that the dc-dc power converter should not drain the output capacitor prior to or at turn-on. Ideally, the output voltage should increase monotonically from the pre-charge voltage value to the final target output voltage. However, this can be a problem for dc-dc power converters employing a synchronous rectifier, especially when the power converter has a soft-start feature. Capacitor draining can potentially occur when a synchronous rectifier switch is enabled to conduct. During soft start, a reference voltage, typically employed to regulate an output voltage of the power converter, starts from 0 volts and ramps up to a final target voltage. Since the output voltage substantially follows the ramping reference voltage, the output capacitor will initially be discharged to substantially zero volts by the synchronous rectifier.
Several approaches have been used to address the problem of power converter start-up with a pre-charged output capacitor and synchronous rectifiers. One approach generally referred to as “diode emulation,” such as used in the Intersil™ Synchronous Rectifier MOSFET Driver ISL6608, senses the current in a low-side synchronous rectifier. If current is detected flowing through the synchronous rectifier from the output capacitor to ground, the synchronous rectifier is disabled. Alternatively, current in the output inductor can be sensed. When the current in the output inductor reaches zero, a low-side switch (i.e., synchronous rectifier switch) is turned off, which prevents the output inductor from sinking current. Nonetheless, either approach requires a relatively accurate current-sensing scheme with a fast response. Such circuits are hard to realize, particularly when using multi-megahertz (“MHz”) switching frequencies.
A second approach, such as described in U.S. Patent Application Publication No. 2004/0169498, entitled “Apparatus for and Method of Adjusting a Switching Regulator Output for a Circuit having a Pre-Charge Voltage,” published Sep. 2, 2004, which is incorporated herein by reference, compares the reference voltage (which controls the output voltage of the power converter) with the output capacitor pre-charge voltage. If the reference voltage is smaller than the pre-charge voltage across the output capacitor, both the main switch and the synchronous rectifier switch are turned off. When the reference voltage ramps up to the pre-charge voltage, a pulse-width modulation (“PWM”) control circuit is enabled. This approach is readily usable with a digital controller. When using analog controllers, however, there is a delay between enabling the controller and the output of an error amplifier reaching the correct voltage. Switching from a high-impedance mode to PWM mode is therefore not smooth, resulting in a transient in the output voltage.
A third approach, such as described in U.S. Pat. No. 7,038,514, entitled “Start-up Circuit for a DC-DC Converter,” issued May 2, 2006, which is incorporated herein by reference, compares an error amplifier output voltage with a preset reference voltage, which is set approximately at the middle voltage of a sawtooth voltage waveform coupled to a comparator that controls the switches of the power converter. In this approach, when the power converter is initially powered on, the error amplifier output is initially low and the switches are off. Once the error amplifier output exceeds the predetermined threshold, the power converter switching action is turned on. Since the threshold is not adjusted for changes in power converter input and output voltages, the controller duty cycle is often not properly matched and, therefore, does not ensure a monotonic ramp of the output waveform.
A fourth approach, such as used in the TPS54673 high-frequency power converter manufactured by Texas Instruments, disables reverse conduction in a synchronous rectifier switch during start-up. Synchronous rectifier switches are turned on only when an error amplifier output saturates at its maximum value. This approach initially turns on the main switch, and eventually both the main switch and the synchronous rectifier switch are switched on. This approach has the same disadvantage as the third approach described above.
Accordingly, what is needed in the art is a process and related method to monotonically increase the output voltage of a PWM-controlled power converter, particularly for a power converter employing synchronous rectifiers, from a pre-charge voltage across an output capacitor to a final target output voltage, that overcomes deficiencies in the prior art.